Cortical projections of the non-entorhinal hippocampal formation in the cynomolgus monkey (Macaca fascicularis)

Episodic memory consolidation requires the integrity of the anatomical pathways between the cerebral cortex and the hippocampal formation. Whilst the largest cortical output of the hippocampal formation originates in the entorhinal cortex, direct projections from CA1, subiculum and presubiculum to the cortex have been reported. The aim of this study is the assessment of the extent, topography and relative strength of those projections, as a parallel/alternate route of memory processing. A total of 45 injections in 28 Macaca fascicularis monkeys were used. Cortical deposits of fluorescent tracers (20 cases, 3% Fast Blue, 2% Diamidino Yellow) or 1% WGA-HRP (eight cases) were made in different cortical areas of the frontal, temporal and parietal lobes, as well as cingulate cortex by direct exposure of the cortical surface. After appropriate survival, animals were perfused and the brains serially sectioned at 50 mum and the retrograde labelling charted with an X-Y digitizing system. Retrograde neuronal labelling was observed in CA1, subiculum, presubiculum and parasubiculum; it was absent in the dentate gyrus, CA3 and CA2. Compared to other portions of the hippocampal formation, the CA1-subiculum border had the highest number of labelled neurons (especially after deposits in the rostral perirhinal cortex), followed by medial frontal cortex, temporal pole, orbitofrontal, anterior and posterior cingulate cortices, parietal and inferotemporal cortices, and no labelling after posterior inferotemporal and lateral frontal cortices. Our results indicate that CA1, subiculum, presubiculum and parasubiculum send direct output to cortical areas. This nonentorhinal, hippocampal formation cortical output may be relevant in memory processing.